About anvil modelling problem caused by mirror modifier
Here's the answer to the problem.  First, grab the center loop and move it towards the center till the clipping does not allow it to go any further.  This makes sure that all of the vertices on the center loop are at the center of the model.  Next, you must move the mirror modifier up above the subdivision surface modifier.  This is a general rule of thumb that will remove the seam problem.
Glad to be of help!